With the increased use of Computer Aided Design (CAD) solid modeling systems a need has developed to translate the CAD output data into a structural component. Forming objects automatically in three dimensions is useful in testing for input CAD errors, part functionality, assessing aesthetics, mold formation by subtractive wax, and small production runs. While some of these applications are somewhat insensitive to short and long range dimensional errors, such as the assessing of aesthetics, other applications are moderately sensitive to error, such as testing part functionality. Still other applications, such as mold manufacturing, are extremely sensitive to dimensional errors.
Automated three dimensional part "printing" techniques that are currently available exhibit generally poor long range dimensional tolerance. A best dimensional tolerance of any presently published technique is approximately one mil over 500 mils, achieved through a binder-droplet-jet-on alumina-powder technique. However, this system was demonstrated only with a simple cartesian grid structure.
While many currently available prototype printing tools are able to make structures with features down to 5-10 mils, the long range change in dimensional accuracy and stability is approximately 5%. The classic test of the dimensional accuracy of a three dimensional prototyping tool is the "T-party". A t-shaped structure having vertical and horizontal bars several inches long is fabricated. The dimensions of the structure are then checked immediately after fabrication, after curing, after several days, and finally after several months. Typically, not only does the length of the bars change by a few percent at each measurement, but the amount of bow in the top bar changes with time as well.
One commercially available system employs software to slice a computer generated solid model, represented by CAD data, into thin cross sections. The cross sections are then physically created by scanning a spot of ultraviolet laser light over a top surface of a reservoir or bath of liquid photopolymer. The scanned laser spot partially cures the photopolymer, changing it from a liquid to a solid. After forming a given layer a supporting stage is lowered within the both by the thickness of the layer created. The scanning process is repeated for the next layer until the structure is completed. After fabrication a subsequent step is typically required to fully cure all of the photopolymer that may be trapped within the partially cured material. Dimensional changes to the structure may occur during this subsequent photopolymerization step.
The following three U.S. patents all teach aspects of photopolymer systems: U.S. Pat. No. 4,565,330, issued Mar. 11, 1986, entitled "Apparatus for Production of Three-Dimensional Objects by Stereolithography" (Hull); U.S. Pat. No. 4,752,498, issued Jun. 21, 1988, entitled "Method and Apparatus for Production of Three-Dimensional Objects by Photosolidification" (Fudim); and U.S. Pat. No. 4,801,477, issued Jan. 31, 1989, entitled "Method and Apparatus for Production of Three-Dimensional Objects by Photosolidification" (Fudim).
Another type of commercially available system employs a laser to sinter a thin layer of powder into the desired shape of each layer.
In general, the photopolymerization and sintering systems are relatively expensive and require a significant amount of time to generate a finished part of average complexity from the input CAD data.
Another commercially available system employs a heated nozzle to extrude a melted material such as nylon wire or a wax. The nozzle is translated under the control of a computer system in accordance with previously sliced CAD data.
The following U.S. Patents are of interest in the field of computer or CAD-defined three dimensional structure fabrication. In U.S. Pat. No. 4,915,757, issued Apr. 10, 1990, entitled "Creation of Three-Dimensional Objects" Rando teaches the machining of a part by laser ablation. In U.S. Pat. No. 4,665,492, issued May 12, 1987, entitled "Computer Automated Manufacturing Process and System" Masters teaches part fabrication by spraying drops or particles. U.S. Pat. No. 4,857,694, issued Aug. 15, 1989, entitled "Method and Apparatus for Automatic Vapor Cooling When Shape Melting a Component" to Doyle et al. relates to cooling shape melted parts. In U.S. Pat. No. 4,890,235, issued Dec. 26, 1989, entitled "Computer Aided Prescription of Specialized Seats for Wheelchairs or other Body Supports" Reger et al. teach the deformation of a surface to create a contour. And finally, in U.S. Pat. No. 4,844,144, issued Jul. 4, 1989, entitled "Investment Casting Utilizing Patterns Produced by Stereolithography" Murphy et al. disclose a method of investment casting utilizing a pattern produced by stereolithography.
What is not taught by this prior art, and what is thus an object of this invention to provide, is a CAD "printing" system for producing a three dimensional structure from CAD data, the system being achieved at relatively low cost while providing high dimensional accuracy, with respect to systems of the prior art.
It is a further object of the invention to provide deposition feedback for providing a closed-loop computer-controlled CAD-generated object fabrication system.
It is another object of the invention to provide methods of generating support structures and anti-aliasing features for a CAD-defined object to facilitate the computer controlled fabrication thereof.